A. Quéméneur

Scaling laws of density fluctuations at high-k on Tore Supra

Anomalous transport in tokamaks is generally attributed to turbulent fluctuations. Since a large variety of modes are potentially unstable, a wide range of short-scale fluctuations should be measured, with wavenumbers from kρi ~ 0.1 to kρi 1. In the Tore Supra tokamak, a light scattering experiment has made possible fluctuation measurements in the medium- and high-k domains where a transition in the k-spectrum is observed: the fluctuation level decreases much faster than usual observations, typically with a power law S(k) ≡ k−6. A scan of the ion Larmor radius shows that the transition wavenumber scales with ρi around kρi ~ 1.5. This transition indicates that a characteristic length scale should be involved to describe the fluctuation nonlinear dynamics in this range. The resulting very low level of fluctuations at high-k does not support a strong effect of turbulence driven by the electron temperature gradient. For this gyroradius scan, the characteristics of turbulence also exhibit a good matching with predictions from gyro-Bohm scaling: the typical scale length of turbulence scales with the ion Larmor radius, the typical timescales with a/cs; the turbulence level also scales with ρi, according to the mixing length rule.

Localized measurements of turbulence in the TORE SUPRA tokamak

A collective infra-red laser scattering diagnostic has been installed on the TORE SUPRA tokamak for the measurement of plasma density fluctuations. For the range of wavenumbers explored (3-15 cm-1), the scattering angles are very weak ( approximately 1 mrad). Consequently, the scattering signals are averaged along the whole observation chord, resulting in poor longitudinal spatial localization. By virtue of the pitch angle variation of the magnetic field lines in the tokamak, and of the perpendicularity of the turbulence wavevector to these field lines, it has been possible to obtain partial spatial resolution along the direction of the beam. Good agreement between the experimental and theoretical angular resolution of the diagnostic as well as the results of cross-correlation performed on the signals obtained by two simultaneous probing beams also justify this novel concept.

ALTAIR: An infrared laser scattering diagnostic on the TORE SUPRA tokamak

A collective laser light scattering diagnostic ALTAIR (a french acronym for local analysis of anomalous transport using infrared light), using a CO2 laser beam (λ=10.6 μm) has been realized to measure plasma density fluctuations in the TORE SUPRA tokamak. This article describes in detail the optical setup, the signal processing, acquisition, and control systems required for this experiment. As the density fluctuations propagating in tokamaks have small wave numbers and require small scattering angles, such scattering experiments are considered as having no resolution along the beam. However, taking advantage of the pitch angle variation of the magnetic field lines around the magnetic axis along a vertical chord, it has been possible to obtain partial spatial localization of the scattering volume by rotating the direction of the analyzed wave vector in a horizontal plane. Heterodyne detection is used to determine the fluctuations propagation direction. The experiment has been tested on acoustic waves and t...

Doppler backscattering system for measuring fluctuations and their perpendicular velocity on Tore Supra

Backscattering of a microwave beam launched in oblique incidence makes possible measurement of density fluctuations close to the cut-off with a selected wave number k⊥=−2ki, where ki is the beam wave vector at the reflection layer. On the system installed on Tore Supra, the incidence of the Gaussian beam is controlled thanks to a tiltable monostatic antenna. The microwave part of the system is based on a fluctuation reflectometer scheme with heterodyne detection, and the choice of a V band (50–75 GHz) microwave source and O mode polarization is appropriate for typical enhanced plasma regimes (n0=3–7×1019 m−3). Both the scattering wave number k⊥ and the scattering localization r/a can be changed during a shot, owing to the steppable probing frequency and the motorized antenna (tilt angle 0–10°). The wave-number range k⊥ is 4–15 cm−1, with a wave-number resolution around 2 cm−1, and the localization r/a∼0.3–0.85. The Doppler effect also provides the perpendicular velocity profile for the same position ra...

Turbulence and energy confinement in TORE SUPRA Ohmic discharges

Results on confinement and turbulence from a set of Ohmic discharges in TORE SUPRA are discussed. Attention is focused on the saturation of the energy confinement time and it is emphasized that this saturation can be explained by a saturation of the electron heat diffusivity. The ion behaviour is indeed governed by dilution and equipartition effects. Although the ion heat transport is never neoclassical, there is no enhanced degradation at saturation. This behaviour is confirmed by turbulence measurements using CO2 laser coherent scattering. The level of density fluctuations follows the electron heat diffusivity variations with the average density. Waves propagating in the ion diamagnetic direction are always present in turbulence frequency spectra. Thus, the saturation cannot be explained by the onset of an ion turbulence. The existence of ion turbulence in the edge at all densities cannot be excluded. However, this ion feature in scattering spectra could be explained by a Doppler shift associated with an inversion point of the radial electric field at the edge

Fluctuation spectra and velocity profile from Doppler backscattering on Tore Supra

Backscattering of a microwave beam close to the cut-off allows for measurement of density fluctuations at a specified wave-number, selected by the scattering geometry , where ki is the beam wave-number at the reflection layer. On the Doppler reflectometry system installed on Tore Supra, both the scattering wave-number k⊥ and the scattering localization (r/a) can be changed during the shot owing to the steppable probing frequency and the motorized antenna. Operating in O mode, the spatial and wave-number ranges depend essentially on density profile, typically probing 0.5 < r/a < 0.95 and 2 < k < 15 cm−1. Wave number spectra are similar to those obtained with conventional scattering systems. The perpendicular fluctuation velocity in the laboratory frame is obtained from the Doppler shift of the frequency spectrum Δω = k⊥v⊥. It is dominated by the plasma Er × B velocity. In the core, the latter is mainly due to the projection of the toroidal velocity, as this is shown by comparison with measurements by charge exchange recombination spectroscopy. In the set of analysed Tore Supra ohmic and ICRH plasmas, the observed rotation is consistent with a poloidal velocity in the electron diamagnetic direction and/or a toroidal velocity in the counter current direction. The detailed structure of the velocity profile, at the edge and in different plasma regimes, allows us then to get information on the radial electric field distribution. The dynamics of the fluctuation velocity can be studied from the time frequency analysis of the signal, for investigating intermittent behaviour and transient regimes.

Quasi-optical Gaussian beam tracing to evaluate Doppler backscattering conditions

Microwave beam backscattering near the cut-off layer appears to be the most interesting diagnostic to observe density fluctuation time evolution for a given localization in the plasma and at a defined wave vector. It also provides perpendicular plasma velocity. Scattering only occurs when the Bragg selection rule is fulfilled, i.e. when the scattering wave vector is almost perpendicular to the magnetic field. In order to evaluate these scattering conditions, ray tracing is required. 3D geometry is necessary to evaluate the angle between the magnetic field and the wave vector at the reflection. The ripple effect on the iso-index layer curve cannot be neglected. Scattering localization and wave vector resolution can be approached if single ray tracing is replaced with quasi-optical beam tracing. Optical propagation is still considered in the WKB approximation but the beam is described as multiple connected rays. The beam radial expansion due to diffraction is well described. This approach allows one to compute beam parameters for all data acquisitions (50 triggers per shot) and all shots (40 shots per day) during the following night on a recent personal computer with MatLab©.

Edge turbulence during ergodic divertor operation in Tore Supra

We report new measurements of turbulence during ergodic divertor (ED) operation. At low density, some de-correlation of the turbulence is observed with a decrease of the long timescale structures. It is shown that the typical time involved is compatible with a de-correlation mechanism through radial separation of the B field lines by the ED, with an associated parallel length of the order of the distance between two modules of the ED. This observation reinforces the conclusion drawn in [1] and based on computer simulations. The situation changes when the density is increased: the turbulence level is found to increase. At the highest density, the structure of the turbulent signal is modified and the bursty behaviour suppressed by the ED at low density reappears. These observations lead to the conclusion that the turbulence measured at high density is not sensitive to the ED stabilization effect. This indicates that it could be carried by the ions.

Statistical study of density fluctuations in the Tore Supra tokamak

It is believed that radial anomalous transport in tokamaks is caused by plasma turbulence. Using infra-red laser scattering techniques on the Tore Supra tokamak, statistical properties of the density fluctuations are studied as a function of the scales in ohmic as well as additional heating regimes using the lower hybrid or the ion cyclotron frequencies. The probability distributions are compared to a Gaussian in order to estimate the role of intermittency which is found to be negligible. The temporal behaviour of the three-dimensional spectrum is thoroughly discussed; its multifractal character is reflected in the singularity spectrum. The auto-correlation coefficient is shown to reflect a short-time coherence of the dissipative structures as well as their long-time incoherence and statistical independence. We also put forward the existence of fluctuations transfer between two distinct but close wavenumbers. A rather clearer image is thus obtained about the way energy is transferred through the turbulent scales.

Temporal separation of the density fluctuation signal measured by light scattering

On Tore Supra, the frequency spectra of the turbulent fluctuations often have two peaks centred on some positive and negative values. These two peaks correspond to a poloidal motion of the electrons in opposite directions. In this paper, a criterion is elaborated which allows us to distinguish, in time, the density fluctuations convected in the parallel direction from those in the anti-parallel direction with respect to the analysing wavevector. Two signals are thus extracted out of one. The validity of our model is experimentally checked by comparing the auto-correlation coefficients and the frequency spectra computed for the whole and the separated signals. Consequently, the frequency spectrum is studied in detail as a function of the analysing wavenumber leading to an accurate determination of some plasma properties.